The present application relates to temperature controlled railroad freight cars, and particularly to railroad freight car body structures incorporating air duct arrangements for circulation of air from a refrigeration or heating unit to various locations within a body of such a car while maximizing available cargo space.
Temperature controlled railroad boxcars are well known, and have long used mechanical refrigeration and heating units mounted on an end wall, primarily to deliver chilled air to the interior of the car. For simplicity, the term refrigeration unit will be used herein to refer to refrigeration units, heating units, or units capable of both heating and cooling. Air from a refrigeration unit is typically forced into one end of an upper plenum extending longitudinally overhead, near the roof of the car, to deliver the conditioned air throughout the car to maintain a desired temperature throughout the cargo space in the car body. Such plenums in the past have intruded down into otherwise useable cargo space more than is desired, in order to assure sufficient air flow throughout the car body. This downward projection has also made the plenum vulnerable to damage from lift trucks moving cargo within such cars.
Typically, an air circulation pattern in such a temperature-controlled car includes flow of air down from the upper plenum onto and along the sides of the cargo and the end wall of the car that is remote from the refrigeration and heating unit. Air returns along the floor to a return air intake plenum leading back up along the near end wall to the refrigeration unit.
As railroad car sizes have increased it has become increasingly difficult to ensure even distribution of air throughout a railroad freight car, as needed in order to avoid uneven cooling that could damage parts of a sensitive cargo. A factor contributing to such difficulty is the desire to provide as much useable cargo space as possible within a boxcar whose size is limited by clearance along rights-of-way where the car is intended to be used.
Another factor in the design of such railcars is the need to avoid excessive car weight, which would limit the weight of cargo that could be carried and add to the cost of fuel used in hauling the car.
In view of these factors, it is desired to provide the necessary air circulation flow and distribution through an upper plenum that is no larger than necessary, so that it takes as little as possible of the potential cargo space within a refrigerated boxcar body, is out of the way of lift truck uprights and the like, and is not unnecessarily heavy.
Along with larger cars has come the desire to use larger lift trucks to quickly load and unload such cars. Lift trucks now in such use are rated at up to 60,000 lb (27240 kilograms) per axle. It is therefore also desired to provide for such a car a floor structure that provides sufficient strength and aids efficient air circulation and thermal conduction to or from the cargo, and yet does not contribute excessive weight to the car.